The present invention concerns a vacuum assisted dewatering box for use in a papermaking machine, such as a Uhle box, a felt suction box, or other types of suction boxes which assist in dewatering the sheet and the fabric upon which it is conveyed in the papermaking machine. In particular, the invention is directed to a dewatering box cover, wherein the cover is comprised of a plurality of block type components which are assembled in a desired manner.
During the process of making paper in a modern papermaking machine, a highly aqueous slurry of about 99% water and about 1% cellulosic fibers is ejected at high velocity either onto an endless moving forming fabric in a single fabric forming arrangement, or in between two converging forming fabrics in a two fabric layout. The fabric or fabrics will pass over one or more vacuum assisted dewatering boxes, typically called a suction box in the fourdrinier section of a papermaking machine, to assist in water removal and consolidate the slurry into a nascent sheet. Upon leaving the forming section, the newly formed sheet has a very high water content of about 75-80%, the remainder being solids. The embryonic sheet is then transferred to the press section where it contacts at least one press fabric which carries it through one or more press nips where further water is pressed from the sheet by mechanical means and passes into the press fabric. The press fabric passes over at least one vacuum assisted dewatering box, typically referred to as a Uhle box in the press section, where water and contamination is removed from the fabric. The sheet, which now typically has a moisture content of about 45-35% continues into the dryer section where the remainder of its water is removed by evaporative means.
Vacuum assisted dewatering boxes are also utilized in other, similar continuous processes, such as in the manufacture of multi-ply boards. In these processes, the sheet is formed in layers and the fabric(s) carry the sheet through several presses where it is dewatered and eventually dried. Vacuum assisted dewatering boxes are employed in the press sections of these machines as well, where the fabric and the product being conveyed upon it must also be dewatered as in the papermaking process.
The vacuum assisted dewatering boxes used in papermaking and like machines have typically been provided with a ceramic cover, to resist the abrasive wear caused by the passage of the fabric and product over its surface. A straight slot extends in the CD across the width of the cover and across the width of the fabric has been effective in providing even drainage. The slot sizes range in linear MD width from about ⅜ inch to about 3.0 inches (1-7.5 cm). However, it has been found that this type of slot arrangement is unsatisfactory in certain instances, such as when a seamed press fabric passes over the slot. The fabric makes a loud popping sound as the seam flap (which is that portion of the batt and base fabric which is extended over the seam area to prevent or inhibit seam marking) is pulled down into the slot. This also causes premature wear at the seam, thus reducing fabric life.
It is known that one means of reducing or significantly eliminating these aforementioned deficiencies of the slot type suction box cover is to utilize one having a herringbone, zigzag or intermittent slot design. The term “herringbone” as used herein in connection with a suction box cover is understood to describe a discontinuous or non-linear slot opening, and this term is also commonly used in the same manner in the industry. These types of covers have been shown to be effective in reducing seam wear by providing more support for the press fabric seam as the fabric moves over the openings. See for example Gatke U.S. Pat. No. 2,957,522, Hood et al. EP 410556, and Bartelmuss et al. U.S. Pat. No. 4,909,906. For the most part, these herringbone covers have not been available in a ceramic design as there was not an economical means of producing them. It will be appreciated by those of skill in the art that it is extremely difficult and costly to machine these very tough ceramic materials so as to provide the desired herringbone type slot opening. A ceramic design with a serpentine cover has been used but it does not provide equal open area across the felt width.
Some suction box covers are presently molded from a plastic material, usually UHMW (Ultra High Molecular Weight) polyethylene. The slots in the covers are routed to form the herringbone or non-continuous slot. The problem with these UHMW covers is that they wear quickly on higher speed machines resulting in increased loss of production due to the need to change the covers more frequently, and potentially increased damage to the press felts due to uneven fabric wear, particularly at the seam.
Unfortunately, the typical methods for manufacturing this type of cover in ceramic have not been either cost effective, or simply could not be used to produce the necessary configuration.
The present invention seeks to overcome these problems and provide a novel, economical means of constructing a ceramic herringbone type suction box cover for use in a papermaking or similar machine. The novel cover provides improved wear life due to its ceramic surface construction, and a non-continuous slot arrangement so as to increase fabric wear life by reducing wear at the seam. It would also be desirable to provide a construction method that allows for a reduced manufacturing cost, even when working with the desired ceramic materials for the covers.